Modernize the codebase

[sunesimonsen]

Fixes: #210

[coveralls]

Pull Request Test Coverage Report for Build 865

• 201 of 210 (95.71%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 92.177%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/matchesSelector.js	1	4	25.0%
src/index.js	200	206	97.09%

Totals
Change from base Build 855:	0.6%
Covered Lines:	520
Relevant Lines:	547

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💛 - Coveralls

[papandreou]

Are you using lebab for the grunt work?

[papandreou]

This has served me well recently:

for transform in arrow arrow-return let obj-shorthand obj-method ; do for dir in lib test; do lebab --transform $transform --replace $dir; done ; done

[sunesimonsen]

Are you using lebab for the grunt work?

Yes :-)

[sunesimonsen]

@papandreou there was a good amount of magicpen usage that wasn't using the output argument, so that needed to be fixed manually.

It is a joy to refactor a codebase with this amount of tests.

[sunesimonsen]

Most of this PR is automated rewriting, but I also have to do some manual work to get there. I count of the tests to validate the solution.

[papandreou]

Looks good to me! There's also a few places where we can use template strings: #225